1. Field of the Invention
The present invention relates to compositions that reduce reflection of exposing radiation from a substrate back into an overcoated photoresist layer. More particularly, the invention relates to antireflective coating compositions that can be applied as coating layers that are highly conformal with respect to an underlying substrate.
2. Background Art
Photoresists are photosensitive films used for transfer of an image to a substrate. A coating layer of a photoresist is formed on a substrate and the photoresist layer is then exposed through a photomask to a source of activating radiation. The photomask has areas that are opaque to activating radiation and other areas that are transparent to activating radiation. Exposure to activating radiation provides a photoinduced chemical transformation of the photoresist coating to thereby transfer the pattern of the photomask to the photoresist coated substrate. Following exposure, the photoresist is developed to provide a relief image that permits selective processing of a substrate.
A photoresist can be either positive-acting or negative-acting. For most negative-acting photoresists, those coating layer portions that are exposed to activating radiation polymerize or crosslink in a reaction between a photoactive compound and polymerizable reagents of the photoresist composition. Consequently, the exposed coating portions are rendered less soluble in a developer solution than unexposed portions. For a positive-acting photoresist, exposed portions are rendered more soluble in a developer solution while areas not exposed remain comparatively less developer soluble. Photoresist compositions in general are known to the art and described by Deforest, Photoresist Materials and Processes, McGraw Hill Book Company, New York, ch. 2, 1975 and by Moreau, Semiconductor Lithography, Principles, Practices and Materials, Plenum Press, New York, ch. 2 and 4, both incorporated herein by reference for their teaching of photoresist compositions and methods of making and using the same.
A major use of photoresists is in semiconductor manufacture where an object is to convert a highly polished semiconductor slice, such as silicon or gallium arsenide, into a complex matrix of electron conducting paths, preferably of micron or submicron geometry, that perform circuit functions. Proper photoresist processing is a key to attaining this object. While there is a strong interdependency among the various photoresist processing steps, exposure is believed to be one of the more important steps in attaining high resolution photoresist images.
Reflection of activating radiation used to expose a photoresist often poses limits on resolution of the image patterned in the photoresist layer. Reflection of radiation from the substrate/photoresist interface can produce spatial variations in the radiation intensity in the photoresist, resulting in non-uniform photoresist linewidth upon development. Radiation also can scatter from the substrate/photoresist interface into regions of the photoresist where exposure is not intended, again resulting in linewidth variations. The amount of scattering and reflection will typically vary from region to region, resulting in further linewidth non-uniformity.
Reflection of activating radiation also contributes to what is known in the art as the xe2x80x9cstanding wave effectxe2x80x9d. To eliminate the effects of chromatic aberration in exposure equipment lenses, monochromatic or quasi-monochromatic radiation is commonly used in photoresist projection techniques. Due to radiation reflection at the photoresist/substrate interface, however, constructive and destructive interference is particularly significant when monochromatic or quasi-monochromatic radiation is used for photoresist exposure. In such cases the reflected light interferes with the incident light to form standing waves within the photoresist. In the case of highly reflective substrate regions, the problem is exacerbated since large amplitude standing waves create thin layers of underexposed photoresist at the wave minima. The underexposed layers can prevent complete photoresist development causing edge acuity problems in the photoresist profile. The time required to expose the photoresist is generally an increasing function of photoresist thickness because of the increased total amount of radiation required to expose an increased amount of photoresist. However, because of the standing wave effect, the time of exposure also includes a harmonic component which varies between successive maximum and minimum values with the photoresist thickness. If the photoresist thickness is non-uniform, the problem becomes more severe, resulting in variable linewidths.
Variations in substrate topography also give rise to resolution-limiting reflection problems. Any image on a substrate can cause impinging radiation to scatter or reflect in various uncontrolled directions, affecting the uniformity of photoresist development. As substrate topography becomes more complex with efforts to design more complex circuits, the effects of reflected radiation become more critical. For example, metal interconnects used on many microelectronic substrates are particularly problematic due to their topography and regions of high reflectivity.
With recent trends towards high-density semiconductor devices, there is a movement in the industry to shorten the wavelength of exposure sources to deep ultraviolet (DUV) light (300 nm or less in wavelength), KrF excimer laser light (248.4 nm) and ArF excimer laser light (193 nm). The use of shortened wavelengths of light for imaging a photoresist coating has generally resulted in increased reflection from the upper resist surface as well as the surface of the underlying substrate. Thus, the use of the shorter wavelengths has exacerbated the problems of reflection from a substrate surface.
Another approach used to reduce the problem of reflected radiation has been the use of a radiation absorbing layer interposed between the substrate surface and the photoresist coating layer. See, for example, PCT Application WO 90/03598, EPO Application No. 0 639 941 A1 and U.S. Pat. Nos. 4,910,122, 4,370,405 and 4,362,809, all incorporated herein by reference for their teaching of antireflective (antihalation) compositions and the use of the same. Such layers have also been referred to as antireflective layers or antireflective compositions or xe2x80x9cARCsxe2x80x9d.
While it has been found that prior antireflective compositions may be effective for many antireflective applications, prior compositions also may pose some potential performance limitations, e.g. when the antireflective compositions are used with resist compositions to pattern features of sub-micron or sub-half micron dimensions.
More particularly, antireflective compositions are perhaps most frequently applied over topography where substrate reflections can be especially problematic. Significant topography will exist with a wide variety of microelectronic device substrates. For example, field oxide isolation and trench isolation fabrication techniques produce significant topography, including vertical and sloping steps, over which photoresist compositions are applied and processed.
However, current antireflective compositions often do not form uniform coating layers over such topography. For example, current ARCs applied by preferred methods of spin coating tend to pool in valley regions and drain off the high points and edges. This can result in additional etch time to clear the valley regions, ineffective amounts of ARC present on the high points and edges and variations in substrate reflectivity due to departure of the ARC coating layer thickness from the desired quarter wave thickness that can provide optimal anti-reflection properties.
It thus would be desirable to have new antireflective coating compositions. It would be particularly desirable to have antireflective compositions that can be applied as highly conformal coating layers, even when applied over substrate topography.
The present invention provides new light absorbing compositions suitable for use as antireflective coating compositions (xe2x80x9cARCsxe2x80x9d), particularly for deep UV applications.
The antireflective compositions of the invention in general comprise a resin binder component that contains a high molecular weight polymer, e.g. a polymer having an Mw of at least about 30,000 daltons, more preferably an Mw of at least about 40,000 daltons, sill more preferably an Mw of at least about 50,000, 60,000 or 70,000 daltons. Polymers having an Mw of at least about 80,000, 90,000, 100,000, 110,0000, 120,000, 130,000, 140,000, 150,000, 160,000, 170,000, 180,000 or 190,000 daltons also will be suitable for use in the ARCs of the invention.
It has been surprisingly found that ARCs of the invention exhibit exceptional conformality upon application to a substrate surface. For example, ARCs of the invention can coat substantial topography such as vertical and sloping steps quite uniformly, i.e. a coating layer of substantially constant thickness across the topography.
Such substrate topography will exist in the manufacture of any of a number of microelectronic devices. For example, sloping step profiles will be formed by locol oxidiation of silicon (also known as xe2x80x9cLOCOSxe2x80x9d) treatments. Trenches will be formed by growing oxide layers between transistor areas, or other isolation techniques employed in device manufacture.
A number of approaches can be employed to incorporate the high molecular weight polymer into an ARC of the invention.
For example, the high molecular weight polymer can be the predominant component of the ARC resin binder, i.e. the high molecular weight polymer can comprise about 90 or 95 percent or more of polymer present in the ARC, excluding any crosslinker or acid generator compounds.
Alternatively, the ARC resin binder component can comprise a polymer blend where only a portion of the blend consists of high molecular weight materials. Thus, for example, extremely high polydispersity polymers which contain high molecular weight species can be employed. Such high polydispersity polymers can be readily prepared e.g. by blending different lots of polymers having significantly different Mw or Mn. It is also not necessary that a blend partner be an identical polymer.
The ARCs of the invention are preferably crosslinking compositions, i.e. one or more components of such an antireflective composition is capable of some type of reaction that crosslinks or otherwise hardens the applied coating layer. Such crosslinking-type compositions preferably comprise an acid or acid generator compound to induce or promote such crosslinking of one or more components of the ARC. Generally preferred crosslinking antireflective compositions comprise a separate crosslinker component such as an amine-based material. The invention also includes antireflective compositions that do not undergo significant cross-linking during intended use with a photoresist composition. Antireflective compositions of the invention are suitably used with both positive-acting and negative-acting photoresist compositions.
The invention further provides methods for forming a photoresist relief image and novel articles of manufacture comprising substrates coated with an antireflective composition of the invention alone or in combination with a photoresist composition. Other aspects of the invention are disclosed infra.